THE BIOCHEMISTKY OF THE THYKOID GLAND 259 



solution content. The iodin content is 65 per cent, the solubility one part 

 in 84,000, keto form. 



Osterherg is stated to have synthetized a small quantity of thyroxin in 

 1917 and efforts are at present he ing made in the Mayo laboratories to 

 reproduce the synthesis on a larger scale. 



Summing up, it would appear that only two definite compounds con- 

 taining iodin have been isolated from thyroid tissue, a. globulin (iodothy- 

 roglobulin) existing as such in the gland, and a derivative of tryptophan 

 (thyroxin), obtained as a cleavage product, and containing 65 per cent of 

 iodin. Thyroxin accounts for one-fourth of the total iodin content of the 

 gland. lodothyroglobulin has a variable iodin content ranging from a 

 trace to 1.7 per cent. This iodin content varies with that of the gland and 

 probably accounts for practically all of it ; otherwise the compound has a 

 definite composition. It has not yet been shown that thyroxin occurs 

 amongst the cleavage products of the globulin, nor is any definite informa- 

 tion available as to the form in which the remaining three-fourths of the 

 iodin exists in the cleavage products. 



Evidence Connecting the Iodin Compounds of the 

 Gland with the Internal Secretion 



General Discussion. The internal secretion appears to leave the gland 

 through vascular channels. Carlson and Woelfel (1910) showed that the 

 rate of lymph production in normal thyroids is very small, but is much 

 increased in goiters though no iodin is present. They criticize the earlier 

 view that the colloid passes to the lymph spaces. Watts (1915) showed 

 that nervous stimulation (cervical sympathetic) produces vasoconstriction 

 in the gland, and this, together with decrease of blood flow produced 

 mechanically, is accompanied by a decrease of iodin and water content, 

 which are considered to leave through the vascular channels. Bensley's (6) 

 results (1916) indicate that the secretion exists in the cells as a dilute 

 solution resembling the colloid, and is probably excreted directly through 

 the vascular channels. Marine and Feiss (1915) state that it is possible 

 to wash out with defibrinated blood a very small amount of idothyroglo- 

 bulin. On a priori grounds this seems very unlikely ; it is to be expected 

 that only simple compounds such as thyroxin will pass into the blood 

 stream. 



So far there is no definite test for the internal secretion of the thyroid. 

 We are, in a measure, forced to test any preparation considered to be the in- 

 ternal secretion or its active ingredient indirectly by a comparison with 

 desiccated thyroid itself, using as tests the action on goiter (which is also 

 produced by various forms of iodin administration), the comparative toxic 

 effect of large doses, the specific effect on myxedema and cachexia thyreo- 



